Conference calls have turned out to be a widely used service, replacing ad hoc meetings between participants separated from each other. Conference calls have been available in circuit switched PSTN networks for a long time, mainly based on speech. However, for a conference call to be a satisfactory substitute to a personal meeting, it has to offer additional multimedia services like video and data streaming. PSTN networks were originally constructed to support speech only, and are therefore not well suited for transferring multimedia services. Thus, IP networks will instead mainly be utilized as bearers of multimedia conferencing in the future.
3GPP [1] is a IP based standard specifying a node called MRF (Media Resource Functions). The task of the MRF is to handle multimedia conferencing between two or more end-points in a packet switched network and i.a. to mix the data streams which belongs to the same conference. Both ad-hoc and scheduled conferences may be set up with the MRF as a centralised conference bridge, mixing the different media streams.
A more general term for MRF is Conference Unit. In this document Conference Unit is referred to as MRF giving a more concrete view of the problems and the corresponding solutions set forth. The aspects are not limited to MRF, but are addressed to any Conference Unit according to the SIP protocol.
There are two main problems with the above mentioned approach of the 3GPP. First, the MRF may run out of resources. This may occur when too many participants are taking part in each conference, and/or when too many conferences are handled simultaneously by the same MRF, leading to an excess of the capacity limit. This may cause in a refusal of additional participants or conferences requesting an overloaded MRF, or the already existing may be damaged through lowered quality or processing speed.
Even if an MRF contains sufficient capacity to handle large conferences, still another problem may occur when transmitting all the associated media streams to one central mixer. This may create a large network load, leading to slow and/or damaged data transmission.
These problems also occur in other SIP [2] networks supporting centralised conferencing using a component similar to the MRF.
Use of a multi-processor system may solve the first problem. This may involve continuous monitoring of the processor load of the MRF, and then simply adding more processor capacity when needed. However, multi-processor systems are very expensive, and will never scale indefinitely. In addition, all available processor recourses will not be utilized optimally, as large conferences may be accumulated in one MRF leaving another unused. Multi-processor solutions will not solve the transmit and receive problem described in the section above.
Another well-known solution is to arrange MRFs hierarchically in a tree, where the different terminals joining the conference will actually join different MRFs. This is (among other places) described in [3].
Using a pre-defined hierarchy of MRFs is a very static solution, and also requires the endpoints to be configured to use one specific MRF. A dynamic solution is rather to prefer, because the nature of such solutions distributes the transmission of data and execution of tasks on communication links and processor recourses, respectively, in a more optimal manner.